Auto merge of #182 - jonhoo:fast-drop-empty, r=Amanieu
Auto merge of #182 - jonhoo:fast-drop-empty, r=Amanieu

Do not iterate to drop if empty

If the table is empty, there is no need to iterate over all the buckets
in order to drop their contents.

Fixes #170.

rust-lang/hashbrown,rust-lang/hashbrown